1. Field of the Invention
The present invention relates to a toner for developing electrostatic latent images and method of manufacturing same.
2. Description of the Related Art
Heretofore, monocomponent developing methods and two-component developing methods have been used as electrostatic latent image developing methods. In both methods toner is charged and delivered by electrostatic force to develop an electrostatic latent image. Disadvantages arise in these methods, however, insofar as when the toner is inadequately charged, not only is the amount of toner charge reduced, but toner possessing an opposite charge is also generated, leading to toner adhesion in non-image regions which causes fog in the produced image.
For this reason, charge controllers are conventionally added to toner to impart desired triboelectric charging characteristics to said toner. Various useful charge controllers include nigrosine dyes, quaternary ammonium salts, monoazo complex with chromium, salicylic acid complex with chromium and the like. Although toners which include the aforesaid charge controllers initially exhibit excellent charging characteristics, these characteristics are reduced after repeated use and result in other disadvantages such as filming and fogging.
Japanese Laid-Open Patent Application Nos. 1-306861, 3-63664, and 3-112990 disclose measures to eliminate the previously described disadvantages by using organic boron compounds as charge controllers. These compounds are advantageous in that they have excellent heat stability, are colorless, or effectively appear colorless in color tone, and are therefore suitable for use in color toners.
When manufacturing toners containing organic boron compounds by kneading pulverization methods, however, disadvantages arise insofar as the mixture is subjected to severe temperature elevation even when cooled during mixing by cooling the mixing machine (e.g., Henschel mixer) while processing the material prior to kneading. When the temperature of the mixture exceeds a constant temperature, the binding resin becomes molten and is difficult to remove from the mixing device, such that remixing is required after mixing must be stopped and the mixture adequately cooled. A disadvantage therefore arises insofar as production characteristics are markedly reduced. Inadequate mixing results when the mixing time is shortened and the mixing process ended before temperature elevation occurs. When the material is inadequately mixed, a mixture containing a uniform dispersion of constituent materials cannot be obtained, and further disadvantages arise insofar as the toner obtained when this mixture is pulverized and classified after fusion kneading causes filming and fogging.
From the perspectives of consumption of raw materials, environmental characteristics, and economics of manufacturing toner by the aforesaid kneading pulverization method, U.S. Pat. No. 5,272,034 proposes the recovery of undesired toner powder produced by the pulverization and/or classification processes (i.e., fine toner particles excluded from the commercial toner) and adding this toner powder to the material in the mixing process as reusable toner material. When toner powder is added to the material being mixed, said toner powder normally works as a buffering agent which makes it difficult for the temperature of the mixture to become elevated, but in the case of organic boron compounds, the addition and mixing of toner powder causes the aforesaid disadvantages to occur as a result of a marked rise in temperature of the mixture.